Leaky Muncie

Hello, I was wondering if anyone could give me some advice on my leaky transmission. I know I said make and model is a firebird however I’m fairly confident this transmission is out of a 1965 chevelle if that makes any difference. It is a Muncie m20 Totally unknown mileage.

Oil is coming from literally everywhere. From between the bell housing and the transmission, from inside the bell housing, from the shifter seals, from the rear main and every gasket. I have to constantly put oil into it every few drives.

Right now I have the transmission out of the car and I’m not sure what to do with it. Mostly I want to know if it’s worth it to try and rebuild or if I should buy a new one or what. I don’t necessarily have a lot of funds but I also don’t need a perfect transmission just one that doesn’t dump oil. However if it’s going to cost as much to rebuild it as just getting a new transmission I’d rather not waste my time. Thanks for any advice.

It isn’t terribly hard to replace all the seals if the trans was working OK but leaky. You should be able to find a video or instructions online.

The side cover, the tailshaft housing and input shaft retainer need come off.

I would bet it will be difficult for you to find someone who can rebuild it.

Make sure there’s nothing restricting the breather vent on the transmission.


Because if there is, and as the transmission and gear oil heat up, and the pressure increases inside the transmission builds, it’ll blow gear oil out of every seal/gasket on the transmission.



There aren’t that many seals and gaskets in a manual transmission, If you can get it out you can probably replace them with no problem. You’re not ‘rebuilding’ the transmission, just replacing the seals.

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My memory has grown a bit dim over the years, but I do seem to remember those Muncie “rock crushers” having some issues with case porosity.

Video tutorial Part 1 right here…


Do you mean you have to top off the transmission every few drives? Or are you referring to the engine oil? If the engine’s rear main seal is leaking it could cause engine oil to go all over the inside of the bell-housing.

This is great, although it might give me the false courage to completely rebuild it. Thank you

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Transmission oil, sorry for not clarifying. The oil on the clutch cover smells like gear oil too and there’s oil all over front of the transmission

It’s possible the only thing that’s leaking is the transmission input shaft seal. You might try replacing that, check to make sure the unit is properly vented, and cross your fingers. You might be able to figure out a way to spin it (with a drill, etc), and visually check if there’s any obvious leak spots. (I’ve never tried that myself, but it seems like it would be possible since your transmission is out of the vehicle, on the bench.)

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